Kem Sokha had been charged with "colluding with foreigners" under Article 443 of Cambodia's penal code, the Phnom Penh Municipal Court said in a statement.

"The act of secret collusion with foreigners is an act of treason," it added.

The evidence the Government has presented is a video of Kem Sokha from 2013 in which he tells supporters of his Cambodia National Rescue Party (CNRP) that he has had American support and advice for his political strategy to win power.

One of the opposition leader's lawyers, Pheng Heng, said what appeared in the video clip was no crime.

"The legal procedure is wrong and the charge isn't correct," he said.

"His words are educational in a workshop ... What he talked about was elections in a multi-party democratic way."

Lawyers met Kem Sokha on Monday for 20 minutes and said his health was fine.

"I may lose freedom, but may freedom never die in Cambodia," Kem Sokha was quoted as saying in a post on Twitter that was repeated by his daughter, Monovithya Kem.